Do not try to look up "Internet Censorship" or "SOPA" or "PIPA" on Wikipedia, the giant online encyclopedia, on Wednesday. SOPA and PIPA are two bills in Congress meant to stop the illegal copying and sharing of movies and music on the Internet, but major Internet companies say the bills would put them in the impossible position of policing the online world.

Wikipedia's founder, Jimmy Wales, says his site will go dark for the day on Wednesday, joining a budding movement to protest the two bills.

"This is going to be wow," Wales said on Twitter. "I hope Wikipedia will melt phone systems in Washington on Wednesday. Tell everyone you know!"

Several sources said members of Congress, reacting to the online objections, were pulling back on parts of SOPA and PIPA to which Internet companies object. But the protest movement continued for the time being.

Other sites, such as Reddit and Boing Boing, have already said they would go dark on Wednesday. And some of the biggest names online, including Google, Facebook, Twitter and Tumblr, have vocally opposed the proposed legislation, though they have not said they are joining the online blackout.

Google today said it would highlight the issue with a link Wednesday on its home page in the U.S. "Like many businesses, entrepreneurs and web users, we oppose these bills because there are smart, targeted ways to shut down foreign rogue websites without asking American companies to censor the Internet," said a Google spokesperson.

PIPA, the Protect IP Act in the Senate, and SOPA, the Stop Online Piracy Act, have been presented as a way to protect movie studios, record labels and others. Supporters range from the Country Music Association to the U.S. Chamber of Commerce.

But the Internet giants say the bills could require your Internet provider to block websites that are involved in digital file sharing. And search engines such as Google, Yahoo and Bing could be stopped from linking to them -- antithetical, they say, to the ideal of an open Internet.

"If you want an Internet where human rights, free speech and the rule of law are not subordinated to the entertainment industry's profits, I hope you'll join us," said Cory Doctorow of Boing Boing.

Wikipedia, the sixth most visited site in the world, said its English version will be dark for 24 hours Wednesday, urging users to contact Congress. Other joiners of the movement include Mozilla, which offers the Firefox Web browser; the Wordpress blogging site; and TwitPic, which allows Twitter users to post images online.

The House bill is on hold for now, and Sen. Patrick Leahy (D.-Vt.), who sponsored the Senate version, said he would be in favor of further research on provisions that have raised objections from Internet service providers. The White House over the weekend said it had reservations about the approach the two bills take.

"While we believe that online piracy by foreign websites is a serious problem that requires a serious legislative response, we will not support legislation that reduces freedom of expression, increases cybersecurity risk, or undermines the dynamic, innovative global Internet," wrote three White House managers, including Aneesh Chopra, the U.S. Chief Technology Officer.

"Any effort to combat online piracy must guard against the risk of online censorship of lawful activity and must not inhibit innovation by our dynamic businesses large and small."

It has become a battle pitting Hollywood against Silicon Valley -- movie studios and music publishers want to stop the theft of their creative work, but Internet companies do not want to be cast as the police force.

"There isn't one technology company or venture capitalist who supports these bills," said Markham Erickson, the executive director of NetCoalition, a trade group for Internet firms, in an interview with ABC News.

"An 'Internet blackout' would obviously be both drastic and unprecedented," NetCoalition said in a statement. "We hope that the Senate will cancel its scheduled vote on PIPA so that we can get back to working with members on how to address the concerns raised by the MPAA [Motion Picture Association of America] and others without threatening our nation's security or future innovation and jobs."

The heads of major Internet companies say they grant that music publishers and Hollywood studios have a real problem: People are stealing their music and movies, making digital copies that are as crisp and clear as the originals, and offering them for download, often from overseas websites.

The music and film industries say they consider that a major threat, even a decade after Napster made online file sharing a major issue.

"More than 2.2 million hard-working, middle-class people in all 50 states depend on the entertainment industry for their jobs and many millions more work in other industries that rely on intellectual property," Michael O'Leary of the Motion Picture Association of America said in a statement. "For all these workers and their families, online content and counterfeiting by these foreign sites mean declining incomes, lost jobs and reduced health and retirement benefits."

But the devil is in the details, said NetCoalition's Erickson.

"This bill reverses the policy that has been in place since the beginning of the Web," he said, "that Internet companies shouldn't be liable, nor should they be required to police or snoop on their users."